The invention relates to a domestic appliance comprising a thermoelectric heat pump, which contains at least one series arrangement of Peltier elements that can be driven with a constant supply voltage, the resistance of which elements increases with increasing temperature and for which a threshold temperature for the Peltier elements is specified.
From a document that was downloadable on Nov. 25, 2005 from the internet the construction and method of operation of a Peltier element having the previously described construction is known. The half-elements of this Peltier element are formed as columns or blocks and consist of doped semiconductors as the active materials. The semiconductors are specifically bismuth telluride and a p-type doped and an n-type doped semiconductor are employed. A half-element of p-type doped semiconductor and a half-element of n-type doped semiconductor are each connected together on one side by a small circuit board, also known as a metal bridge and on another side, opposite the stated one side, each connected by a further small circuit board to a further half-element or to a connection contact for connection of the Peltier element to an electrical network.
Further information on the principles, application-related selection and assembly of Peltier elements are available from documents that were downloadable on 25 Nov. 2005.
DE 1 410 206 A refers to a domestic appliance in the form of a washing machine in which the articles of washing are not only washed but can also be dried. The document shows a number of alternatives for the additional equipment required for this; in particular an electrical heating device for heating a flow of air for drying the washing and a simple heat exchanger for cooling the warmed flow of air after its contact with the washing can be provided, but the heater and the cooler can also be parts of a heat pump. The heat pump can be a heat pump that works with Peltier elements to make use of the thermoelectric effect; in this case the heat pump is called a “thermoelectric heat pump”.
A domestic appliance for the drying of washing referred to in an English abstract of JP 08 057 194 A from the collection “Patent Abstracts of Japan” has, in a system of ducts, in addition to a heater and a cooler, both of which are part of a thermoelectric heat pump, an additional heat exchanger connected upstream of the cooler for cooling the air flow from the items of washing and a further heating device connected downstream of the heater for further heating the air flow before it impinges on the items of washing.
A domestic appliance in the form of a tumble dryer with a thermoelectric heat pump is known from DE 69 26 182 U. Normally the resistance of all Peltier elements increases with increasing temperature. In the case of Peltier elements a threshold temperature is preset by the use of solder that can melt. For Peltier elements a threshold temperature of 150° C. is usual.
Such a construction can cause a problem when there is a fault in the operation of the domestic appliance, for example a failure of a blower fan, excessive drying of the washing or an excessive amount of lint in the air passages. If appropriate safety measures are not provided in such a case, the heat pump can be destroyed by overheating. When such overheating occurs, a solder joint in a Peltier element can be damaged and thereby its electrical contact properties affected; it is also possible that metal diffuses from an overheated solder joint into a semiconductor material of the Peltier element, causing a long-term impairment to its operation.
In a tumble dryer or an appropriately equipped washing machine, washing is normally dried in a drum to which hot air is supplied.
The hot air becomes saturated with evaporating water and the resulting humid air is guided toward the thermoelectric heat pump. On a cold side of this heat pump water condenses out of the humid air and is eliminated from it. The air is guided toward a hot side of this heat pump where it is reheated in order to be supplied again to the drum. A fault can occur in this operation. For example, it can occur that, as a result of blockage in the airways such as by items of washing or accumulations of lint, sufficient air no longer flows past the thermoelectric heat pump. The abnormal operating condition can also arise that the tumble dryer should stop on its own because the washing is dry or the drum is empty but the tumble dryer continues to run.
In each of these cases the Peltier elements of the thermoelectric heat pump become heated above their normal operating temperature, which is usually between 60° C. and 100° C. It is easily possible that their threshold temperature is exceeded, resulting in a failure of the modules and furthermore a certain risk of fire can arise.